For industrial, commercial and domestic water heaters, combustible fuel is often used as a heat source. For example, natural gas has been a preferred choice of fuel.
A simple arrangement for a water heater is to place a burner below a to tank filled with water. The combusted hot gases are allowed to flow around the tank so that the water within will absorb heat from the combusted gases. As water heaters developed, an opening was placed through the center of the water storage tank so that combusted gases could pass both around the outside and through the center of the tank, giving more surface area for absorption of heat from the combusted gases. The pathway or hole through the center of the tank became known as the flue tube.
Eventually, water heaters were placed within enclosed areas; for example, a domestic water heater may be placed in the basement of the home. This created the need for the combusted gases to be exhausted under control, namely through exhaust ventilation systems. The need for ventilated exhaust systems encouraged the use of flue tubes and discouraged the passage of exhaust gases around the water storage tank.
Modern water heaters, therefore, have virtually eliminated passing combusted gases around the outside of the water tank. Instead, modern water heaters generally direct combusted exhaust through a central flue tube. Eliminating passing exhausted gases outside of the water storage tank reduced the surface area from which the water in the tank could potentially absorb heat from the combusted exhaust. Additionally, a flue tube placed within a water heater tank directly above the combusted gases allows the hot combusted gases to more quickly flow through the water tank without transferring heat.
There have been attempts to design a flue tube to allow for greater heat transfer from the combusted gases across the wall of the flue tube and into the water of the water storage tank. For example, U.S. Pat. No. 4,660,541 issued to Moore is directed to a water heater with a submerged combustion chamber. A baffle is positioned in the flue tube along its length and causes turbulence in the exhausted gases as they flow upward through the flue tube.
U.S. Pat. No. 4,677,939 issued to Henault et al. is directed to a heat exchanger for a fluid heating apparatus, particularly a domestic hot water accumulator. The flue tube in Henault et al. is a hermetically sealed tube disposed spirally in the lower third of an enclosure so that the different turns of the ringed tube are tangentially in contact with one another in order to form a compact exchanger.
Reissue Patent No. RE37,240 issued to Moore, Jr. et al. is directed to a water heater with reduced localized overheating. Moore, Jr. et al. teaches a small pump to circulate the water within the tank when the burner is activated so that any water separated into layers of different temperature will be mixed.
U.S. Pat. No. 4,157,706 issued to Gaskill is directed to a flue tube fitted with a turbulator means which disperses entering hot gases along the surface of the flue's inner wall. In the water tank, Gaskill provides another turbulator means which disperses water along the interior surface of the inner and outer water tank walls. The turbulator means is a continuous metal ledge situated in a spiral configuration along the tank's inner wall.
Despite the foregoing proposals, there remains a need for a simple flue tube design that helps to facilitate the transfer of heat from the combusted exhaust gases to the water in the water storage tank.